Physics Department Colloquium
A single Rydberg electron in a Bose-Einstein Condensate
Time: 3:30 PM
Place: 2241 Chamberlin Hall (coffee at 430 pm)
Speaker: Prof. Tilman Pfau, University of Stuttgart
Abstract: Electrons attract polarizable atoms via a 1/r^4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length , which was first introduced to explain their findings.
At ultracold temperatures and Rydberg states with medium size principle quantum numbers n, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules . These molecules can show a linear Stark effect corresponding to a permanent dipole moment , which if seen from a standpoint of traditional molecular physics is surprising.
At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers n in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, leading again to a density dependent energy shift of the Rydberg state. This allows, together with the strong van-der-Waals blockade, to excite only one single Rydberg atom in a condensate. We excite a Rydberg electron with n upto 202 in the BEC, the size of which becomes comparable to the size of the BEC. We study their life time in the BEC and the coupling between the electron and phonons in the BEC . So the single electron that we prepare in a quantum gas allows nicely to study the transition from two- to few- to many-body interaction.
As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number and the imaging of the Rydberg electron's wavefunction by its impact onto the surrounding ultracold cloud seem to be within reach.
 E. Amaldi and E. Segre, Nature 133, 141 (1934)
 C. H. Greene, et al. PRL. 85, 2458 (2000); V. Bendkowsky et al., Nature 458, 1005 (2009)
 W. Li, et al., Science 334, 1110 (2011)
 J . B. Balewski, A. T. Krupp, A. Gaj, D. Peter, H. P. Büchler, R. Löw, S. Hofferberth, T. Pfau, Nature 502, 664 (2013)